1. Field of the Invention
The present invention relates to the design and method of construction of lightweight building modules and to methods of installing such modules at a building site.
2. Description of the Prior Art
A complete modular building system using a prefabricated lightweight building module comprising thin reinforced concrete wall and ceiling panels attached to a floor slab is described in U.S. Pat. No. 3,990,193, issued on Nov. 9, 1976, of which the applicant herein is a joint inventor.
In the preferred module design disclosed in that patent, each ceiling panel and wall panel includes a thin, continuous layer of concrete having spaced apart open web bar girders embedded in one surface thereof. Each bar girder is a welded assembly having a continuous zig-zag bent bar forming an open web extending between one side edge comprising two straight round bars welded to opposite sides of alternate bends of the zig-zag bar and another side edge comprising two straight angle bars welded to opposite sides of the bend intermediate the first-mentioned alternate bends.
Each of the open web reinforcing bar girders of the side wall and ceiling panels of the patent terminate in a flat connection plate, and when the panels are assembled with a matching floor slab, one edge of each ceiling girder connection plate is butt welded to a contiguous edge of the upper connection plate of the corresponding side wall girder.
The floor slabs shown in the patent have no similar open web reinforcing bar girders. Instead, preshaped reinforcing steel bars are placed in forms and tied together in the conventional manner prior to pouring the concrete for the floor slabs. At spaced intervals along the opposite sides of each floor slab flat plates are welded to the adjacent reinforcing bars to provide steel surfaces to which the adjacent vertical edges of the lower connecting plates of the corresponding side wall reinforcing girders can be welded.
Under field assembly conditions it has been found that obtaining sufficiently accurate alignment of the upper side wall girder connection plates with the corresponding ceiling girder connection plates to permit butt welding the adjacent edges of the two plates is difficult and time consuming. Furthermore, bending moments exerted by racking forces on the module place such butt welds in tension, which is undesirable in view of the relatively low tensile strength of the deposited weld metal.
The same problem is inherent in the edge welds between the lower connection plates of the side wall girders and the side plates of the floor slab. In addition, the design of the floor slab provides no continuity from one side to the other for distributing the moment forces exerted through the connection of each side wall girder to the floor slab.